The present invention relates to methods for determining flow rates of substances by temperature readings effected by remote sensing such as thermoimaging or contact sensing using a contact thermosensor. More particularly, the present invention relates to methods for calculating flow rates of substances within objects by extracting and mathematically manipulating numerical parameters from transient temperature response curves of the surface of the objects.
The methods of the present invention are suitable, for example, to determine coronary flow rates and myocardial perfusion rates in open as well as closed chest conditions, that is, to calculate flow rates of substances along the coronary arteries and, myocardial perfusion rates of the substances into the heart tissue, by extracting and mathematically manipulating numerical parameters from transient temperature response curves of the coronary arteries or a specific part of an artery and, of the heart tissue or a specific part of the heart tissue, respectively.
Availability of real-time information regarding coronary flow and myocardial perfusion may be of great value for the cardiac surgeon. For example, during the course of by-pass heart surgery, such data may be exploited for (i) deciding which regions of the coronary arteries are narrowed and, therefore, are to be by-passed; and (ii) checking the successfulness of each by-pass graft.
Methods aimed at flow rates measurements may be divided into two major groups.
The first group of methods aimed at flow rates measurements includes methods in which a detection probe is made in direct contact with the substance, which substance flow being measured, therefore, methods associated with this group are generally termed invasive methods. An example of an invasive method is the thermodilution method, in which an invasive temperature detection probe is made in direct contact with the substance of which flow is measured. In general, the invasive methods suffer three major drawbacks for application in human diagnostics, the first is their invasiveness and, the second is their ability to record data from one or at the most only few locations at a given time, whereas the third is the requirement for direct contact with the flowing substance.
The second group of methods aimed at flow rates measurements include non-invasive methods, therefore, methods associated with this group are generally termed imaging methods.
Various non-invasive imaging methods were developed for different applications in human diagnostics as well as in other fields. A common feature characterizing these methods is the use of a contrast agent, which agent is traced. Imaging methods which are used primarily for diagnostic purposes include, for example, (i) X-ray based imaging methods in which X-rays are used to detect either an internal body anatomy, such as bones in a simple X-ray analysis or, an administrated radiopaque contrast agent (e.g., iodine) used in CT and, other X-ray based imaging methods; (ii) ultrasound based imaging methods in which ultrasonic waves are used to detect either an internal body anatomy in simple ultrasound analysis or, an administrated contrast agent, such as microbubbles, used in contrast-echo. Yet, in other imaging methods implemented in medicine and other fields, radioactive materials are employed as detectable agents, which materials may be detected by, for example, various kinds of radioactivity counters.
While using imaging methods for flow rate determinations of bodily fluids, such as blood, within the body, the flow rate of the blood containing an external contrast agent provided into the body or into a specific organ in an upstream region is measured.
The methods described hereinabove, in which external contrast agents are traced, suffer a major drawback when employed for medical purposes, since in the course of their application, an external contrast agent, some times poisonous or with yet undetermined commulative effects is administrated to the human body.
Thermoimaging is an imaging method suitable for flow measurements and is devoid of all the above mentioned limitations, when employed for medical purposes. Thermoimaging is an InfraRed waves based method for detecting heat, which is, therefore, employed as a contrast agent, termed hereinbelow a thermo-contrast agent. Since the contrast agent used for thermoimaging is heat, the method is safer to the patient. Thermography is a thermoimaging method generally used for estimate flow rates of bodily fluids, such as blood, employing a thermal (i.e., InfraRed) camera, focused on an examined body organ and, a digital image processor which provides images of the organ with high spatial (ca. 0.75 mm) and thermal (0.1.degree. C) resolution, which images can be displayed on a high resolution monitor, in real-time. When thermography is used to estimate blood flow along the coronary arteries during revascularization surgery (i.e., coronary artery by-pass surgery, CABG) it is termed Thermal Coronary Angiography (TCA). TCA is a technique that is capable of providing unique, clinically relevant information about epicardial coronary arteries and by-pass grafts in real-time during revascularization surgeries. See for example U.S. Pat. Nos. 4,995,398 and 5,375,603.
However, thermography, can be applied only during an open chest procedure, whereas, for obvious inherent reasons, it is not applicable during a closed chest procedure, which recently has become the procedure of choice in many cases.
To this effect the reader is referred to "Endovascular cardiopulmonary bypass and cardioplegia as a basis for port-access cardiac surgery " published in the Official Journal of the Society of Endoscopic & Laparoscopic Surgeons of Asia (ELSA) Vol. 2 No. 3, pp. 52-54, 1997; and "Minimally invasive cardiac surgery: a reality " published in the Official Journal of the Society of Endoscopic & Laparoscopic Surgeons of Asia (ELSA) Vol. 2 No. 3, pp. 56-58, 1997.
Among other applications, TCA is a method that was developed to replace some invasive methods for specific applications, imaging methods exploiting harmful contrast agents, and methods with other, such as, for example, accuracy, limitations, to estimate blood flow through the coronary arteries and by-pass grafts. These methods include arteriography in which a radiopaque dye is injected into the coronary arteries, which dye serves as a contrast agent; passage of coronary probes; electromagnetic flow measurements; angioscopy; and cine-angioscopy. Each of these methods has its specific limitations.
TCA involves injecting 20-30 ml of a cold substance, such as, for example, crystalloid cardioplegia, saline or blood, into, for example, the aortic root and, recording the temperature changes associated with, for example, the surface of the coronary arteries by a thermovision system. In cases where a beating heart is inspected, the temperature changes of the surface of the coronary arteries associated with warm blood replacing the cold substance may be recorded alternatively or additionally. Recordings can also be made of the surface of the heart tissue itself, which recordings reflect perfusion of the cold substance or of body temperature blood, into the heart tissue. Alternatively, if the flow of blood through the coronary arteries is artificially reduced or ceased completely for a given period of time, the epicardial temperature will drop to a minimal level close to the surrounding rooms temperature. Replenishing blood flow of body temperature blood at this point will elevate the epicardial temperature and a transient temperature response curves will, therefore, be available. In this case, as generally stated above, the blood, which is the substance which flow is measured, serves also as the thermo-contrast agent.
Thermography in open-chest conditions have been previously employed also for verification of graft patency (see, Mohr, et al. (1989) Thermal coronary angiography: a method for assessing graft patency and coronary anatomy in coronary bypass surgery. Ann. Thorac. Surg. (USA), 47(3):441-449.) and proper myocardial cooling during cardioplegia (see, Pantaleo et. al. (1984) Thermographic evaluation of myorardial cooling and intraoperative control of graft patency in patients with coronary artery disease. J. Cardiovasc. Surg. 25(6):554-559.). Assessment of blood perfusion by thermography of the myocardium was also reported (see, Adachi and Becker (1987) Assessment of myocardial blood flow by real-time InfraRed imaging. J. Surg. Res., 43(1):94-102, and Kekesi et. al. (1986) Hemodynamics and thermographic signs of intermyocardial venous outflow redistribution induced by coronary sinus occlusion in the canine heart. Acta. Chir. Hung., 27(4):203-15.).
However, in all of these cases, and others, the kinetic (i.e., transient response) of the temperature from successive thermoimages was not deduced.
Furthermore, when TCA is employed in real-time, during by-pass surgeries, estimation of flow through the coronary arteries is based on visual inspection of the coronary tree as reflected by its thermal image, which visual inspection enables to observe a presence of narrow zones or blockages along the coronary arteries. Nevertheless, such visual inspection does not provide numerical data regarding the actual flow of substances along the coronary arteries.
It is an object of the present invention to provide tools for calculating flow rates of substances within objects by extracting and mathematically manipulating numerical parameters from transient temperature response curves of the surface of the objects.